Enhanced performance of nanocomposite membrane developed on sulfonated poly (1, 4-phenylene ether-ether-sulfone) with zeolite imidazole frameworks for fuel cell application

Proton exchange membrane fuel cells (PEMFC) have received a lot of interest and use metal–organic frameworks (MOF)/polymer nanocomposite membranes. Zeolite imidazole framework-90 (ZIF-90) was employed as an addition in the sulfonated poly (1, 4-phenylene ether-ether-sulfone) (SPEES) matrix in order to investigate the proton conductivity in a novel nanocomposite membrane made of SPEES/ ZIF. The high porosity, free surface, and presence of the aldehyde group in the ZIF-90 nanostructure have a substantial impact on enhancing the mechanical, chemical, thermal, and proton conductivity capabilities of the SPEES/ZIF-90 nanocomposite membranes. The results indicate that the utilization of SPEES/ZIF-90 nanocomposite membranes with 3wt% ZIF-90 resulted in enhanced proton conductivity of up to 160 mS/cm at 90 °C and 98% relative humidity (RH). This is a significant improvement compared to the SPEES membrane which exhibited a proton conductivity of 55 mS/cm under the same conditions, indicating a 1.9-fold increase in performance. Furthermore, the SPEES/ZIF-90/3 membrane exhibited a remarkable 79% improvement in maximum power density, achieving a value of 0.52 W/cm2 at 0.5 V and 98% RH, which is 79% higher than that of the pristine SPEES membrane.

The adverse impact of the widespread use of fossil fuels on the environment, specifically with respect to climate change, has resulted in significant efforts to identify and implement feasible and sustainable alternatives. As a result, there is an increasing focus on exploring and utilizing environmentally-friendly renewable energy sources, including hydrogen. One of the energy production systems that utilizes hydrogen fuel is fuel cells 1 . Researchers have taken an interest in the Proton Exchange Membrane Fuel Cell (PEMFC) as a green energy technology among various fuel cells, owing to its distinctive features and benefits. These advantages include high start-up speed, efficiency, and current density, along with a low operating temperature and emission-free operation 2 . Actually, one of the most essential parts of PEMFCs is the proton exchange membrane, which directly determines whether the fuel cell performs successfully or not. Therefore, preparing a suitable membrane for application and accelerating the commercialization process in PEMFC has been one of the main goals of many researchers 3 . A number of non-flourinated polymers, such as sulfonated poly (ether ether ketone) 4 , sulfonated poly (phthalazinone ether ketone) 5,6 , poly vinyl alcohol 7 , and sulfonated poly ether sulfone [8][9][10] , have recently been investigated as alternatives to commercial Nafion. A new family of coordination polymers known as metal-organic frameworks (MOFs) has been identified that is made up of metal clusters attached to organic ligands that have a three-dimensional crystalline structure 11 . MOFs have various applications such as storage, separation, and catalysis and are also used as biological carriers in medicine [12][13][14][15] . Among the various applications, a large number of MOFs have shown good potential for proton and ion conduction [16][17][18] . MOFs have a high proton conductivity due to their highly flexible design, free surface, and high porosity 11,19 . The ZIF belongs to the large family of MOFs and is made by connecting a divalent metal ion (often Zn 2+ ) to four imidazole anionic linkers. It has characteristics like a very www.nature.com/scientificreports/ Synthesize of ZIF-90. The ZIF-90 nanostructure has been synthesized according to the procedure 47 . In summary, in this method, 0.75 mmol of zinc nitrate cluster and 2.10 mmol of 2-imidazole carboxyhydride linker are solved separately in 50 mL and 100 mL of DMF, respectively. In the third step, 1.96 ml of trioctylamine is dissolved separately in 50 mL of DMF solvent at ambient temperature. So the zinc nitrate metal cluster is slowly added to the ICA organic linker. In the final step, trioctylamine is added to the solution. Finally, the product is centrifuged and after washing for several with ethanol solvent and the end is dried in a vacuum oven at 80 °C for 12 h.

Sulfonation of PEES.
According to the reference, SPEES was obtained through the postsulfonation of PEES ( Fig. 1) 48 . In briefly 20 mL of 98% concentrated sulfuric acid, 2 g of PEES polymer is dissolved at room temperature. After 12 h at 25 °C, the solution dissolves on a magnetic stirrer. Then, for extracting the sulfonated polymer, uniform solution is added slowly and dropwise to cold deionized water (containing ice). This action results in the precipitation of the sulfonated polymer. The produced polymer is washed with deionized water to neutralize the pH (pH = 7). The produced polymer is dried in a vacuum oven at 100 °C. The titration method was used to determine the sulfonation degree (DS) of SPEES in this work. The DS was calculated to be around 68%. Characterization. The ZIF-90 nanostructure's successful synthesis was confirmed by FT-IR, XRD, and N 2 adsorption analyses. The BELSORP MINI II adsorption instrument manufactured by Microtrac (Japan) measured the Langmuir surface area, specific Brunauer-Emmett-Teller (BET), pore volume, and pore size distribution. The 8400S model was subjected to Fourier transform infrared spectroscopy (FTIR) analysis (Germany). The X-ray diffraction (XRD) analysis was conducted using the Bruker D8 and GNR Explorer diffractometers from Italy, utilizing Cu Kα radiation. With a resolution of 4 cm −1 and a region of 600-4000 cm −1 , Bruker Equinox 55 was used to perform the ATR-FTIR spectra. The morphology of the SPEES/ZIF-90 nanoocomposite membranes was seen using a TESCAN MIRA 3 field emission scanning electron microscope (FESEM). The morphology-phase atomic force microscopy (AFM) JPK NanoWizard II model manufactured by BRUKER was utilized to examine the membrane morphology. On a LINSEIS, analyses using thermogravimetric analysis (TGA) were carried out under atmosphere at a heating rate of 10 °C/min. DSC analyses were obtained using the Q600 (USA) at a rate of 10 °C/min in a N 2 atmosphere. Mechanical parameters of the dry membranes were used by Santam STM-50 model with the velocity of 10 mm.min −1 . Using a potentiostat-galvanostat Metrohm called the PGSTAT303N, proton conductivity measurements were performed. The conductivity of proton (σ) was obtained from the following relation 50 :

Construction of nanocomposite membranes.
Here L represents the membrane thickness (cm), R is the resistance obtained from the Nyquist curve (ohm), and S is the membrane surface area (cm 2 ). The slope of the Arrhenius plots can be operated to determine the Activation energy (E a ) by following relation: Here, A is the Arrhenius constant, R is gas constant (8.314 J/mol.K) and T was the temperature (Kelvin). www.nature.com/scientificreports/ The water uptake (WU)) is obtained from the difference between dry (W dry ) and wet weight (W wet ) (after 24 h of immersion in water) of the membrane from Eq. (3) that using the method reported in references 50,51 .
The IEC value of the membrane was deter defined mined by the conventional titration method as reported elsewhere 49,50 . where M NaOH was the molar concentration of NaOH solution (0.1 M), V NaOH was the volume of NaOH solution (L) and W M was the weight of a dry sulfonated polymer (SPEES (g)). Degree of SPEES sulfonation depends on the IEC and is described by the following relation 50 .
For investigation the oxidation stability of membranes, Fenton test was done based on the procedure explained by Grot and LeClech 52,53 . The weight loss percentage in membrane can be calculated according to: The creation of membrane electrode assemblies (MEAs) is necessary to investigate the PEMFC's final performance. The catalyst ink is first prepared by dissolving the specified quantity of 20 wt. % Pt-C powder in isopropyl alcohol/water and a SPEES solution. A carbon fiber fabric with a microporous layer and a loading of 0.5 mg/ cm 2 will be painted with catalyst ink. The second step involves drying the prepared electrodes between 80 °C and 120 °C. To create the electrode-membrane assembly, the prepared electrodes and membrane were squeezed at 50 kg/cm 2 for 5 min at 120 °C. Finally, the potential was held constant at 0.5 V for 6 h until the temperature reached 80 °C in order to activate the produced MEAs. Finally at flow rates 300/500 mL/min of hydrogen/Oxygen were inserted into the anode and cathode electrodes.

Results and discussions
Characterization of ZIF-90. Figure  As shown in Fig. 2b, the purity and bonding characteristics of the ZIF-90 structure produced using the FT-IR spectrum are examined. The peaks at 3417 cm −1 and 3282 cm −1 in Fig. 2b are connected to the aromatic stretching vibration's N-H and C-H bonds. The peaks in the region of 1674 cm −1 and 2852 cm −1 are the tensile vibrations of the C = O aldehyde group and the C-H in the aldehyde group, respectively. While the peaks in the region of 1361 cm −1 , 1415 cm −1 , and 1456 cm −1 are related to the C-H, C = C, and C = N flexural vibration of the ring, respectively, the peaks located in the 600-1500 cm −1 region are related to the total tensile or flexural vibrations of the imidazole ring. These peaks confirm the ZIF-90 structure, which is in line with earlier studies 11 .
The nitrogen adsorption and desorption isotherm at − 196 °C (77 K) is depicted in Fig. 2c. Additionally, the measured ZIF-90 nanostructure properties are compiled in Table 1 and include the BET contact surface, pore volume, and pore diameter. The present study reports a measured BET surface area of 1180 m 2 /g for ZIF-90. The adsorption/desorption isotherms exhibit a classification of Type I according to IUPAC standards. This indicates that the primary pores of the adsorbent substance fall within the micro range. A review of the data demonstrates that ZIF-90's N 2 adsorption/desorption isotherm accurately reveals the structure of the sample that was synthesized using the available sources 11,19 . The crystal structure of ZIF-90 is also displayed in  In Fig. 3b indicates the X-ray diffraction pattern of the SPEES, SPEES/ZIF-90/3, SPEES/ZIF-90/5 and SPEES/ ZIF-90/7 membranes. The broad crystalline peak in the XRD pattern is visible in the 2θ = 19° (related to the SO 3 H  www.nature.com/scientificreports/ group) for the SPEES membrane, which corresponds to the relevant reference 54 . As shown in the Fig. 3b, the broad peak is visible in all membranes. The intensity of peak width is reduced by increasing the ZIF-90 content in SPEES/ZIF-90/x nanocomposite membranes. This may be due to the presence and effect of ZIF-90 nanostructure on SPEES membranes. On the other hand, the presence of ZIF-90 in SPEES/ZIF-90/x nanocomposite membranes with 2θ = 7° and 2θ = 12° peaks has been shown 47 . In Fig. 4, exhibits the cross-sectional images of the FESEM-AFM corresponding to the SPEES/ZIF-90/3 and SPEES/ZIF-90/5 membranes. Figure 4a shows the FESEM image of the SPEES/ZIF-90/3 nanocomposite membrane, which shows the uniform distribution of ZIF-90 on the basic membrane. The cross-section of the SPEES/ZIF-90/3 has suitable morphology. Figure 4b shows the accumulation of ZIF-90 nanostructure on the surface of SPEES/ZIF-90/5 nanocomposite membrane with 5 wt. % of ZIF-90. Figure 4c The stress-strain relationship between the membranes for SPEES, SPEES/ZIF-90/1, SPEES/ZIF-90/3, and SPEES/ZIF-90/5 is shown in Fig. 5c. The maximum applied tensile strength and the elongation at break for various membranes are also shown in Fig. 5d. The curves show that the force applied to the SPEES/ZIF-90/3 membrane, with a value of 51.385 MPa, results in the greatest resistance. With more ZIF-90 present, however, www.nature.com/scientificreports/ the amount of elongation decreases. These findings demonstrate how the addition of ZIF-90 can significantly enhance the thermal, chemical, and mechanical characteristics of nanocomposite membranes. Differences in chemical stability of different membranes are indicated in Fig. 5e. The results indicate that the rupture time and weight loss versus increasing percentage of ZIF-90. For a 3 wt. % of ZIF-90, the weight lost relative to the SPEES polymer membrane is halved and the rupture time is increased by 2 h, and the claim of increased chemical stability can be proved by the presence of 3 wt. % ZIF-90. Increasing the values by more than 5 wt. % ZIF-90 reduces the chemical stability that may be due to the accumulation of ZIF-90.
Proton conductivity of nanocomposite membranes. The properties of the SPEES membrane and the SPEES/ZIF-90/x nanocomposite membranes were compared in the WU, IEC and proton conductivity.
As shown in Table 2, with increasing the ZIF-90 content to 3 wt. %, the amount of water uptake has increased from 38.61% to 68.79% at the 25 °C and the other tempratures. So that SPEES/ZIF-90/3 nanocomposite membranes is reported with the highest amount of water uptake in the different temprature. In fact, high porosity and surface area and existing aldehyde group of ZIF-90 is caused to trap water molecule in the pores. The reduction in the percentage of water uptake can be attributed to the accumulation of ZIF-90, as evidenced by the increase in its concentration by over 3 wt. %. The IEC of a membrane shows how many acid groups there are in every gram of the sample and how many ionizable functional groups are present in the membrane. According to Table 2, with the increase in ZIF-90 content by 7 wt. %, the IEC has decreased from 1.73 meq/g to 1.589 meq/g. This decrease is due to enhanicng the presence of ZIF-90 nanostructure and reduction of SO 3 H groups and increasing of electrostatic interactions between the polymer acidic group and the ZIF-90 functional group (aldehyde group) [56][57][58] .
The conductivity of Proton is one of the effective parameters for evaluating PEMFC performance. Several elements, including water uptake, IEC, and type of nanoparticles, have an impact on the proton conductivity of nanocomposite membranes. In Fig. 6a shows the proton conductivities of SPEES and their nanocomposite membranes at 25 °C with various percentages of ZIF-90. The proton conductivity of SPEES/ZIF-90/x nanocomposite membranes effectively increases when compared to that of the SPEES membrane, as shown in Fig. 6a. In other words, ZIF-90 is essential for improving the conductivity of protons in nanocomposite membranes. The aldehyde group and imidazole ring also enhance Grotthus' mechanism by facilitating proton transfer at proton hopping sites. Comparing the results, the SPEES/ZIF-90/3 membrane performed better than other membranes with proton conductivities of 105 mS/cm and 75 mS/cm (at 25 °C and 98% and 70% RH, respectively). However, proton conductivity is decreased by blocking proton transport channels at concentrations greater than 5 wt. % ZIF-90. On the other hand, Fig. 6b,c shows the proton conductivity of nanocomposite membranes at various temperatures. The conductivity of protons has increased with temperature because their mobility has improved. SPEES/ZIF-90/3 nanocomposite membranes had conductivities of 105 mS/cm and 160 mS/cm at 25 °C and 90 °C, respectively, according to a comparison of the various nanocomposite membranes. These numbers are greater than the 21 mS/cm and 55 mS/cm proton conductivities of SPEES. This data leads us to assume that the MOFs nanostructure does have a long-term impact on improving proton conductivity on MOF/polymer nanocomposite membranes.
Time-stability is another important parameter in the PEMs. Figure 6d illustrates the proton conductivity lifetime plots of SPEES/ZIF-90/3 membrane at 95 °C and 98% RH. The SPEES/ZIF-90/3 nanocomposite membranes showed stable proton conductivity after 180 h. The SO 3 H group of polymer, -CHO group and imidazole ring of ZIF-90 nanostructure trigger the good hydrogen bonding, trapping the water in the pores and so proton conductivity remains Table.  Table 3 compiles an overview of the literature on Nafion 117 and various sulfonated aromatic polymers' ability to form nanocomposite membranes with proton conductivity. The analysis of the data revealed that the SPEES/ ZIF-90/3 nanocomposite membrane's proton conductivity performed better under the same conditions than the other results mentioned. The increase in water uptake at various temperatures at the membrane's interface, which can lead to stability in the proton transfer pathways, and the even distribution of the ZIF-90 nanostructure are both responsible for this increase. Fig. 7, the current density-potential (I-V) and current density-power density curves of nanocomposite membranes made of SPEES and SPEES/ZIF-90/3 at 70 °C and 90 °C and 70% www.nature.com/scientificreports/   (Fig. 7) had the best performance in terms of polarization curves (160 mS/cm at 90 °C and 98% RH), which may be because it is more capable of absorbing water and conducting protons. One of the key elements that affects how well produced membranes perform in the end is proton conductivity, which rises with increasing relative humidity from 70% RH to 98% RH.

Fuel cell performance. As shown in
Reporting the open circuit voltage (OCV) of the PEMFC for 100 h, as shown in Fig. 7c, allowed for the determination of the long-term stability of the SPEES/ZIF-90/3 nanocomposite membrane at 90 °C and 98% RH. Referring to its high WU (89% at 80 °C) and high mechanical stability, the OCV in the PEMFC constituted by the SPEES/ZIF-90/3 nanocomposite membrane practically maintained a constant quantity after 100 h (tensile strength: 51.385 MPa). The final result was a nanocomposite membrane (SPEES/ZIF-90/3) that performed exceptionally well over an extended period of time.

Conclusion
One of the intriguing and successful possibilities for improving membranes and boosting the effectiveness of polymer membranes in fuel cell performance is the use of metal organic frameworks (MOFs). In this research, we produced a new Polymer/MOF nanocomposite membrane for use in PEMFC by using this technique. In comparison to a SPEES-based membrane, the SPEES/ZIF-90/3 nanocomposite membrane demonstrated superior proton conductivity of up to 160 mS/cm under 90 °C and 98% RH. This enhanced conductivity is believed to be due to the membrane's effective water uptake properties, which are attributed to the ZIF-90 nanostructure. Furthermore, the SPEES/ZIF-90/3 nanocomposite membrane exhibited exceptional thermal, chemical, www.nature.com/scientificreports/ and mechanical stability. The excellent proton conductivity of the SPEES/ZIF-90/3 nanocomposite membrane resulted in improved PEMFC performance at 90 °C compared to the standard SPEES membrane. Consequently, the SPEES/ZIF-90/3 nanocomposite membrane emerged as a promising candidate for PEMFC applications. The membrane's superior water uptake and proton conductivity led to superior PEMFC performance, resulting in current densities and power densities of 1.07 A/cm 2 and 0.52 W/cm 2 , respectively, outperforming the SPEES membrane at 90 °C (Supplementary Information).

Data availability
The datasets used and/or analyzed during this paper are publicly available from corresponding author.